Production of catalyst coated membranes

ABSTRACT

A method for the production of catalyst coated membranes, especially catalyst coated membranes for use in fuel cells, includes raised relief printing a catalyst coating composition ( 11 ) onto the surface of an ion exchange membrane ( 16 ) to form at least one electrode layer.

FIELD OF THE INVENTION

[0001] This invention relates to a method for the production of catalystcoated membranes for use in electrochemical cells, especially catalystcoated membranes for use in fuel cells.

BACKGROUND OF THE INVENTION

[0002] A variety of electrochemical cells falls within a category ofcells often referred to as solid polymer electrolyte (“SPE”) cells. AnSPE cell typically employs a membrane of a cation exchange polymer thatserves as a physical separator between the anode and cathode while alsoserving as an electrolyte. SPE cells can be operated as electrolyticcells for the production of electrochemical products or they may beoperated as fuel cells.

[0003] Fuel cells are electrochemical cells that convert reactants,namely fuel and oxidant fluid streams, to generate electric power andreaction products. A broad range of reactants can be used in fuel cellsand such reactants may be delivered in gaseous or liquid streams. Forexample, the fuel stream may be substantially pure hydrogen gas, agaseous hydrogen-containing reformate stream, or an aqueous alcohol; forexample methanol in a direct methanol fuel cell (DMFC). The oxidant may,for example, be substantially pure oxygen or a dilute oxygen stream suchas air.

[0004] In SPE fuel cells, the solid polymer electrolyte membrane istypically perfluorinated sulfonic acid polymer membrane in acid form.Such fuel cells are often referred to as proton exchange membrane(“PEM”) fuel cells. The membrane is disposed between and in contact withthe anode and the cathode. Electrocatalysts in the anode and the cathodetypically induce the desired electrochemical reactions and may be, forexample, a metal black, an alloy or a metal catalyst supported on asubstrate, e.g., platinum on carbon. SPE fuel cells typically alsocomprise a porous, electrically conductive sheet material that is inelectrical contact with each of the electrodes, and permit diffusion ofthe reactants to the electrodes. In fuel cells that employ gaseousreactants, this porous, conductive sheet material is sometimes referredto as a gas diffusion layer and is suitably provided by a carbon fiberpaper or carbon cloth. An assembly including the membrane, anode andcathode, and gas diffusion layers for each electrode, is sometimesreferred to as a membrane electrode assembly (“MEA”). Bipolar plates,made of a conductive material and providing flow fields for thereactants, are placed between a number of adjacent MEAs. A number ofMEAs and bipolar plates are assembled in this manner to provide a fuelcell stack.

[0005] For the electrodes to function effectively in SPE fuel cells,effective electrocatalyst sites must be provided. Effectiveelectrocatalyst sites have several desirable characteristics: (1) thesites are accessible to the reactant, (2) the sites are electricallyconnected to the gas diffusion layer, and (3) the sites are ionicallyconnected to the fuel cell electrolyte. In order to improve ionicconductivity, ion exchange polymers are often incorporated into theelectrodes. In addition, incorporation of ion exchange polymer into theelectrodes can also have beneficial effects with liquid feed fuels. Forexample, in a direct methanol fuel cell, ion exchange polymer in theanode makes it more wettable by the liquid feed stream in order toimprove access of the reactant to the electrocatalyst sites.

[0006] In electrodes for some fuel cells employing gaseous feed fuels,hydrophobic components such as polytetrafluoroethylene (“PTFE”) aretypically employed, in part, to render electrodes less wettable and toprevent “flooding”. Flooding generally refers to a situation where thepores in an electrode become filled with water formed as a reactionproduct, such that the flow of the gaseous reactant through theelectrode becomes impeded.

[0007] Essentially two approaches have been taken to form electrodes forSPE fuel cells. In one, the electrodes are formed on the gas diffusionlayers by coating electrocatalyst and dispersed particles of PTFE in asuitable liquid medium onto the gas diffusion layer, e.g., carbon fiberpaper. The carbon fiber paper with the electrodes attached and amembrane are then assembled into an MEA by pressing such that theelectrodes are in contact with the membrane. In MEA's of this type, itis difficult to establish the desired ionic contact between theelectrode and the membrane due to the lack of intimate contact. As aresult, the interfacial resistance may be higher than desired. In theother main approach for forming electrodes, electrodes are formed ontothe surface of the membrane. A membrane having electrodes so formed isoften referred to as a catalyst coated membrane (“CCM”). Employing CCMscan provide improved performance over forming electrodes on the gasdiffusion layer but CCMs are typically more difficult to manufacture.

[0008] Various manufacturing methods have been developed formanufacturing CCMs. Many of these processes have employedelectrocatalyst coating slurries containing the electrocatalyst and theion exchange polymer and, optionally, other materials such as a PTFEdispersion. The ion exchange polymer in the membrane itself, and in theelectrocatalyst coating solution could be employed in either hydrolyzedor unhydrolyzed ion-exchange polymer (sulfonyl fluoride form whenperfluorinated sulfonic acid polymer is used), and in the latter case,the polymer must be hydrolyzed during the manufacturing process.Techniques that use unhydrolyzed polymer in the membrane,electrocatalyst composition or both can produce excellent CCMs but aredifficult to apply to commercial manufacture because a hydrolysis stepand subsequent washing steps are required after application of theelectrode. In some techniques, a “decal” is first made by depositing theelectrocatalyst coating solution on another substrate, removing thesolvent and then transferring and adhering the resulting decal to themembrane. These techniques also can produce good results but mechanicalhandling and placement of decals on the membrane are difficult toperform in high volume manufacturing operations.

[0009] A variety of techniques have been developed for CCM manufacturewhich apply an electrocatalyst coating solution containing the ionexchange polymer in hydrolyzed form directly to membrane also inhydrolyzed form. However, the known methods again are difficult toemploy in high volume manufacturing operations. Known coating techniquessuch as spraying, painting, patch coating and screen printing aretypically slow, can cause loss of valuable catalyst and require theapplication of relatively thick coatings. Thick coatings contain a largeamount of solvent and cause swelling of the membrane which causes it tosag, slump, or droop, resulting in loss of dimensional control of themembrane, handling difficulties during processing, and poor electrodeformation. Attempts have been made to overcome such problems for massproduction processes. For example, in U.S. Pat. No. 6,074,692, a slurrycontaining the electrocatalyst in a liquid vehicle such as ethylene orpropylene glycol is sprayed on the membrane while the membrane is heldin a tractor clamp feed device. This patent teaches pretreating themembrane with the liquid vehicle prior to the spraying operation todecrease the swelling problems. However, processes employing suchpretreatment steps are complicated, difficult to control, and requirethe removal of large amounts of the vehicle in a drying operation. Suchdrying operations are typically slow and require either disposal orrecycling of large quantities of the vehicle to comply with applicableenvironmental requirements.

[0010] Accordingly, a process is needed which is suitable for the highvolume production of catalyst coated membrane and which avoids problemsassociated with prior art processes. Further, a process is needed whichis suitable for the direct application of an electrocatalyst coatingcomposition to a membrane in hydrolyzed form which avoids the swellingproblems associated with known processes and which does not requirecomplicated pre-treatment or post-treatment process steps.

BRIEF SUMMARY OF THE INVENTION

[0011] The invention provides a process for manufacturing catalystcoated membrane comprising: preparing an electrocatalyst coatingcomposition comprising an electrocatalyst and an ion exchange polymer ina liquid medium; and raised relief printing said catalyst coatingcomposition onto a first surface of an ion exchange membrane. The raisedrelief printing forms at least one electrode layer covering at least apart of said surface of said membrane. Preferably, the raised reliefprinting technique employed is flexographic printing.

[0012] In a preferred process, the raised relief printing is repeated toform multiple electrode layers covering the same part of the surface ofthe membrane. If desired, the process advantageously provides multipleelectrode layers which vary in composition. In addition oralternatively, the raised relief printing advantageously provides anelectrode layer with a predetermined nonuniform distribution ofelectrocatalyst across the electrode layer.

[0013] The process in accordance with the invention is extremelywell-suited to high volume commercial manufacture of catalyst coatedmembrane. Raised relief printing provides thin, well-distributed layersof the electrocatalyst composition and avoids problems associated withcoating techniques which employ large quantities of solvent. The processis extremely versatile and can provide electrodes in any of a widevariety of shapes and patterns and, if desired, can have electrocatalystor other electrode materials which vary in amount or composition acrossthe electrode, through the thickness of the electrode or both.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a schematic view showing the use of flexographic proofpress equipment to form electrodes on one side of a discrete length ofmembrane in accordance with the present invention.

[0015]FIG. 2 is a schematic view showing a continuous process inaccordance with the invention employing membrane roll stock utilizingthree discrete printing stations to form multiple electrode layers in acontinuous fashion.

DETAILED DESCRIPTION

[0016] This invention provides a process for manufacturing catalystcoated membranes which employs raised relief printing technology toapply electrocatalyst containing coating compositions onto ion exchangemembranes. Of particular interest is such a printing process adapted forpreparing catalyst coated membranes for fuel cell applications.

[0017] Raised relief printing as used herein refers to processes whichemploy any of a variety of types of pre-formed plates which have raisedareas which define the shape or pattern to be printed on a substrate. Inuse in accordance with the present invention, the raised areas of theplate are contacted by and become coated with a liquid electrocatalystcoating composition and then the raised areas are brought into contactwith the ion exchange membrane to deposit the composition onto themembrane. After drying, the shape or pattern defined by the raised areasis thereby transferred to the ion exchange membrane to form an electrodelayer. If desired, the relief printing is advantageously employed toform an electrode that is a build-up of multiple electrode layers.

[0018] In accordance with a preferred form of the present invention,flexographic printing is the raised relief printing method employed.Flexographic printing is a well known printing technique used widely forpackaging applications which employs elastomeric printing plates and isdescribed in the Kirk-Othmer's Encyclopedia of Chemical Technology, 4thedition, 1996, John Wiley and Sons, New York, N.Y., volume 20, pages62-128, especially pages 101-105. Such plates include sheet photopolymerplates, sheets made from liquid photopolymer and rubber printing plates.Especially useful are flexographic printing techniques which usephotopolymer printing plates. The most preferred relief printingtechnique employs solid-sheet photopolymer plates such as thephotopolymer flexographic printing plates sold by E.I. Du Pont deNemours and Company of Wilmington, Del. under the trademark Cyrel®.

[0019] The flexographic method offers considerable benefits in cost,changeover, speed, ease of printing on thin extensible substrates, suchas ion exchange membranes and in the variety of electrodes which can beprinted. The printed area may be of virtually any shape or design, bothregular or irregular, which can be transferred to the plate. Possibleshapes include circles, ovals, polygons, and polygon having roundedcorners. The shape may also be a pattern and may be intricate ifdesired. For example, flexography may be used to print an electrodehaving a shape that coincides with pathway of fuel and oxidant flowfields.

[0020] Multiple applications of the same or different coatings to thesame area are easily accomplished using flexographic printing. Inexisting uses of flexography, it is common to apply multiple colors ofink in close registration and these techniques are well-suited to theprinting of electrodes having overlying multiple layers. The compositionand the amount of coating applied per application may be varied. Theamount of coating applied at each pass may be varied across the coatedarea, i.e., length and/or width. Such variation need not be monotonic orcontinuous. The precision of flexographic printing has the furtheradvantage of being very economical in the use of coating solution, whichis particularly important for expensive electrocatalyst coatings.

[0021] The process of the invention also preferably includes the raisedrelief printing of a catalyst coating composition onto the oppositesurface of an ion exchange membrane to form at least one electrode layercovering at least a part of the opposite surface of the membrane inregistration with the electrode layer first applied to the membrane.Again, the ability of flexographic printing to handle multipleapplications in close registration is useful for this aspect of theinvention.

[0022] In the preferred flexographic printing method in accordance withthe invention using solid-sheet photopolymer flexographic plates,commercially-available plates such those sold under the trademark Cyrel®are well adapted for use in the process. Cyrel® plates are thick slabsof photopolymer uniformly deposited/bonded to 5 to 8 mil poly(ethyleneterephthalate) (PET), then capped with a thin easy-release PETcoversheet. The photopolymer itself is a miscible mixture of about 65%acrylic polymer(s), 30% acrylic monomer(s), 5% dyes, initiators, andinhibitors. U.S. Pat. Nos. 4,323,636 and 4,323,637 disclose photopolymerplates of this type.

[0023] Negatives having images to create the raised areas on the platecan be designed by any suitable method and the creation of negativeselectronically has been found to be especially useful. Upon UV exposurethrough the negative, monomer polymerization occurs in select areas.Following removal of the PET coversheet, unexposed, non-polymerizedmaterial may be removed by a variety of methods. The unexposed areas maybe simply washed away by the action of a spray developer. Alternatively,the non-polymerized monomer may be liquefied by heating and then removedwith an absorbent wipe material. A compressible photopolymer reliefsurface, made to photographic resolution is thus created. This reliefsurface serves to transfer electrocatalyst coating composition from abulk applicator to a print applicator or to the substrate surfaceitself. Formation of an electrode layer occurs by simple wetting coupledwith mechanical compression of the elastomeric plate.

[0024] When rubber printing plates are employed, the pattern may begenerated by known techniques including molding said rubber plate in thedesired pattern or by laser ablation to yield the desired shape orpattern.

[0025] The process of the present invention employs electrocatalystcoating compositions which are adapted for use in the raised reliefprinting process. The compositions include an electrocatalyst and an ionexchange polymer in a suitable liquid medium. The ion exchange polymerperforms several functions in the resulting electrode including servingas a binder for the catalyst and improving ionic conductivity tocatalyst sites. Optionally, other components are included in thecomposition, e.g., PTFE in dispersion form.

[0026] Electrocatalysts in the composition are selected based on theparticular intended application for the CCM. Electrocatalysts suitablefor use in the present invention include one or more platinum groupmetal such as platinum, ruthenium, rhodium, and iridium andelectroconductive oxides thereof, and electroconductive reduced oxidesthereof. The catalyst may be supported or unsupported. For directmethanol fuel cells, a (Pt—Ru)O_(X) electocatalyst has been found to beuseful. One particularly preferred catalyst composition for hydrogenfuel cells is platinum on carbon, 60 wt % carbon, 40 wt % platinum suchas the material with this composition obtainable from E-Tek CorporationNatick, Mass. which, when employed accordance with the proceduresdescribed herein, provided particles in the electrode which are lessthan 1 μm in size.

[0027] Since the ion exchange polymer employed in the electrocatalystcoating composition serves not only as binder for the electrocatalystparticles but also assists in securing the electrode to the membrane, itis preferable for the ion exchange polymers in the composition to becompatible with the ion exchange polymer in the membrane. Mostpreferably, exchange polymers in the composition are the same type asthe ion exchange polymer in the membrane.

[0028] Ion exchange polymers for use in accordance with the presentinvention are preferably highly fluorinated ion-exchange polymers.“Highly fluorinated” means that at least 90% of the total number ofunivalent atoms in the polymer are fluorine atoms. Most preferably, thepolymer is perfluorinated. It is also preferred for use in fuel cellsfor the polymers to have sulfonate ion exchange groups. The term“sulfonate ion exchange groups” is intended to refer to either sulfonicacid groups or salts of sulfonic acid groups, preferably alkali metal orammonium salts. For applications where the polymer is to be used forproton exchange as in fuel cells, the sulfonic acid form of the polymeris preferred. If the polymer in the electrocatalyst coating compositionis not in sulfonic acid form when used, a post treatment acid exchangestep will be required to convert the polymer to acid form prior to use.

[0029] Preferably, the ion exchange polymer employed comprises a polymerbackbone with recurring side chains attached to the backbone with theside chains carrying the ion exchange groups. Possible polymers includehomopolymers or copolymers of two or more monomers. Copolymers aretypically formed from one monomer which is a nonfunctional monomer andwhich provides carbon atoms for the polymer backbone. A second monomerprovides both carbon atoms for the polymer backbone and also contributesthe side chain carrying the cation exchange group or its precursor,e.g., a sulfonyl halide group such a sulfonyl fluoride (—SO₂F), whichcan be subsequently hydrolyzed to a sulfonate ion exchange group. Forexample, copolymers of a first fluorinated vinyl monomer together with asecond fluorinated vinyl monomer having a sulfonyl fluoride group(—SO₂F) can be used. Possible first monomers include tetrafluoroethylene(TFE), hexafluoropropylene, vinyl fluoride, vinylidine fluoride,trifluoroethylene, chlorotrifluoroethylene, perfluoro (alkyl vinylether), and mixtures thereof. Possible second monomers include a varietyof fluorinated vinyl ethers with sulfonate ion exchange groups orprecursor groups which can provide the desired side chain in thepolymer. The first monomer may also have a side chain which does notinterfere with the ion exchange function of the sulfonate ion exchangegroup. Additional monomers can also be incorporated into these polymersif desired.

[0030] Especially preferred polymers for use in the present inventioninclude a highly fluorinated, most preferably perfluorinated, carbonbackbone with a side chain represented by the formula—(O—CF₂CFR_(f))_(a)—O—CF₂CFR′_(f)SO₃H, wherein R_(f) and R′_(f) areindependently selected from F, Cl or a perfluorinated alkyl group having1 to 10 carbon atoms, a=0, 1 or 2. The preferred polymers include, forexample, polymers disclosed in U.S. Pat. No. 3,282,875 and in U.S. Pat.Nos. 4,358,545 and 4,940,525. One preferred polymer comprises aperfluorocarbon backbone and the side chain is represented by theformula —O—CF₂CF(CF₃)—O—CF₂CF₂SO₃H. Polymers of this type are disclosedin U.S. Pat. No. 3,282,875 and can be made by copolymerization oftetrafluoroethylene (TFE) and the perfluorinated vinyl etherCF₂═CF—O—CF₂CF(CF₃)—O—CF₂CF₂SO₂F,perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride) (PDMOF),followed by conversion to sulfonate groups by hydrolysis of the sulfonylfluoride groups and ion exchanging to convert to the acid, also known asthe proton form. One preferred polymer of the type disclosed in U.S.Pat. Nos. 4,358,545 and 4,940,525 has the side chain —O—CF₂CF₂SO₃H. Thispolymer can be made by copolymerization of tetrafluoroethylene (TFE) andthe perfluorinated vinyl ether CF₂═CF—O—CF₂CF₂SO₂F,perfluoro(3-oxa-4-pentenesulfonyl fluoride) (POPF), followed byhydrolysis and acid exchange.

[0031] For perfluorinated polymers of the type described above, the ionexchange capacity of a polymer can be expressed in terms of ion exchangeratio (“IXR”). Ion exchange ratio is defined as number of carbon atomsin the polymer backbone in relation to the ion exchange groups. A widerange of IXR values for the polymer are possible. Typically, however,the IXR range for perfluorinated sulfonate polymer is usually about 7 toabout 33. For perfluorinated polymers of the type described above, thecation exchange capacity of a polymer is often expressed in terms ofequivalent weight (EW). For the purposes of this application, equivalentweight (EW) is defined to be the weight of the polymer in acid formrequired to neutralize one equivalent of NaOH. In the case of asulfonate polymer where the polymer comprises a perfluorocarbon backboneand the side chain is —O—CF₂—CF(CF₃)—O—CF₂—CF₂—SO₃H (or a salt thereof,the equivalent weight range which corresponds to an IXR of about 7 toabout 33 is about 700 EW to about 2000 EW. A preferred range for IXR forthis polymer is about 8 to about 23 (750 to 1500 EW), most preferablyabout 9 to about 15 (800 to 1100 EW).

[0032] The liquid medium for the catalyst coating composition is oneselected to be compatible with the process. It is advantageous for themedium to have a sufficiently low boiling point that rapid drying ofelectrode layers is possible under the process conditions employed,provided however, that the composition cannot dry so fast that thecomposition dries on the relief printing plate before transfer to themembrane. When flammable constituents are to be employed, the selectionshould take into any process risks associated with such materials,especially since they will be in contact with the catalyst in use. Themedium should also be sufficiently stable in the presence of the ionexchange polymer which, in the acid form, has strong acidic activity.The liquid medium typically will be polar since it should be compatiblewith the ion exchange polymer in the catalyst coating composition and beable to “wet” the membrane. While it is possible for water to be used asthe liquid medium, it is preferable for the medium to be selected suchthat the ion exchange polymer in the composition is “coalesced” upondrying and not require post treatment steps such as heating to form astable electrode layer.

[0033] A wide variety of polar organic liquids or mixtures thereof canserve as suitable liquid media for the electrocatalyst coatingcomposition. Water in minor quantity may be present in the medium if itdoes not interfere with the printing process. Some preferred polarorganic liquids have the capability to swell the membrane in largequantity although the amount of liquids the electrocatalyst coatingcomposition applied in accordance with the invention is sufficientlylimited that the adverse effects from swelling during the process areminor or undetectable. It is believed that solvents with the capabilityto swell the ion exchange membrane can provide better contact and moresecure application of the electrode to the membrane. A variety ofalcohols are well-suited for use as the liquid medium.

[0034] Preferred liquid media include suitable C4 to C8 alkyl alcoholsincluding, n-, iso-, sec- and tert-butyl alcohols; the isomeric 5-carbonalcohols, 1, 2- and 3-pentanol, 2-methyl-1-butanol, 3-methyl, 1-butanol,etc., the isomeric 6-carbon alcohols, e.g. 1-, 2-, and 3-hexanol,2-methyl-1-pentanol, 3-methyl-1-pentanol, 2-methyl-1-pentanol, 3-methyl,1-pentanol, 4-methyl-1-pentanol, etc., the isomeric C7 alcohols and theisomeric C8 alcohols. Cyclic alcohols are also suitable. Preferredalcohols are n-butanol and n-hexanol. Most preferred is n-hexanol.

[0035] The amount of liquid medium in the electrocatalyst compositionwill vary with the type of medium employed, the constituents of thecomposition, the type of printing equipment employed, desired electrodethickness, process speeds etc. The amount of liquid employed is highlydependent on viscosity of the electrocatalyst composition that is veryimportant to achieve high quality electrodes with a minimum of waste.When n-butanol is employed as the liquid medium, a coating solidscontent of from about 9 to about 18% by weight is a particularly usefulflexographic printing range. Below about 9% solids, viscosity isundesirably low leading to rapid settling of the catalytic particles,physical leaking from coating applicator “fountain” in standard pressesand undesirably low print deposition weights. Furthermore, at levels ofn-butanol greater than about 91% by weight, undesirable swelling ofperfluorinated sulfonic acid membranes can result. Moreover, above about18 wt % coating solids, the electrocatalyst coating compositions takeson a paste-like consistency with the associated handling problems,irregular plate wetting, etc.

[0036] Handling properties of the electrocatalyst coating composition,e.g. drying performance, can be modified by the inclusion of compatibleadditives such as ethylene glycol or glycerin up to 25% by weight basedon the total weight of liquid medium.

[0037] It has been found that the commercially available dispersion ofthe acid form of the perfluorinated sulfonic acid polymer, sold by E.I.du Pont de Nemours and Company under the trademark Nafion®, in awater/alcohol dispersion, can be used as starting material to prepare anelectrocatalyst containing coating suitable for use in flexographicprinting. The method of preparation involves the replacement of thelower alcohols and water in the commercially available dispersion with aC4 to C8 alkyl alcohol through a distillation process. The result is ahighly stable dispersion of perfluorinated sulfonic acid polymer in a C4to C8 alkyl alcohol with a water content less than 2%, more typicallyless than 0.5%. Solids content can be varied up to 20%. Using thismodified dispersion as base for the electrocatalyst coating composition,the catalytic metal or carbon black supported catalytic metal requiredto form an electrode can be added which yields a coating compositionwith excellent printing properties in the process of the presentinvention.

[0038] In the electrocatalyst coating composition, it is preferable toadjust the amounts of electrocatalyst, ion exchange polymer and othercomponents, if present, so that the electrocatalyst is the majorcomponent by weight of the resulting electrode. Most preferably, theweight ratio of electrocatalyst to ion exchange polymer in the electrodeis about 2:1 to about 10:1.

[0039] Utilization of the electrocatalyst coating technique inaccordance with the process of the present invention can produce a widevariety of printed layers which can be of essentially any thicknessranging from very thick, e.g., 20 μm or more very thin, e.g., 1 μm orless. This full range of thicknesses can be produced without evidence ofcracking, loss of adhesion, or other inhomogeneities. Thick layers, orcomplicated multi-layer structures, can be achieved by utilizing thevery precise pattern registration available using flexographic printingtechnology to provide multiple layers deposited onto the same area sothat the desired ultimate thickness can be obtained. On the other hand,only a few layers or perhaps a single layer can be used to produce verythin electrodes. Typically, 1-2 μm thick layers are produced with eachprinting.

[0040] The multilayer structures mentioned above permit theelectrocatalyst coating to vary in composition, for example theconcentration of precious metal catalyst can vary with the distance fromthe membrane surface. In addition, hydrophilicity can be made to changeas a function of coating thickness, e.g., layers with varying ionexchange polymer EW can be employed. Also, protective orabrasion-resistant top layers may be applied in the final layerapplications of the electrocatalyst coating.

[0041] Composition may also be varied over the length and width of theelectrocatalyst coated area by controlling the amount applied as afunction of the distance from the center of the application area as wellas by changes in coating applied per pass. This control is useful fordealing with the discontinuities that occur at the edges and corners ofthe fuel cell, where activity goes abruptly to zero. By varying coatingcomposition or plate image characteristics, the transition to zeroactivity can be made gradual. In addition, in liquid feed fuel cells,concentration variations from the inlet to the outlet ports can becompensated for by varying the electrocatalyst coating across the lengthand width of the membrane.

[0042] Membranes for use in accordance with the invention can be made ofthe same ion exchange polymers discussed above for use in theelectrocatalyst coating compositions. The membranes can be made by knownextrusion or casting techniques and have thicknesses which can varydepending upon the application and typically have a thickness of 350 μmor less. The trend is to employ membranes that are quite thin, i.e., 50μm or less. The process in accordance with the present in invention iswell-suited for use in forming electrodes on such thin membranes wherethe problem associated with large quantities of solvent during coatingare especially pronounced. While the polymer may be in alkali metal orammonium salt form during the relief printing process, it is preferredfor the polymer in the membrane to be in acid form to avoid posttreatment acid exchange steps. Suitable perfluorinated sulfonic acidpolymer membranes in acid form are available under the trademark Nafion®by E.I. du Pont de Nemours and Company.

[0043] Reinforced perfluorinated ion exchange polymer membranes can alsobe utilized in CCM manufacture by the inventive printing process.Reinforced membranes can be made by impregnating porous, expanded PTFE(ePTFE) with ion exchange polymer. ePTFE is available under thetradename Goretex® from W. L. Gore and Associates, Inc., Elkton Md., andunder the tradename Tetratex® from Tetratec, Feasterville Pa.Impregnation of ePTFE with perfluorinated sulfonic acid polymer isdisclosed in U.S. Pat. Nos. 5,547,551 and 6,110,333.

[0044] While the process of the invention can be performed to makediscrete lengths of catalyst coated membrane with a limited number ofelectrodes on each side of the membrane, the invention is advantageouslycarried out by performing the raised relief printing in a continuousfashion using roll stock.

[0045]FIG. 1 shows the use of flexographic proof press equipment to formelectrodes on one side of a discrete length of membrane in accordancewith the present invention. As shown in FIG. 1, in coating station 10,the electrocatalyst coating composition 11 is picked up by the aniloxroll 12. An anilox roll is a standardized tool of the printing industryconsisting of a precision engraved cellular surfaced roll which drawsout a uniform wet ink film from the ink reservoir. The wet ink thicknessis controlled by the specific anilox cell geometry chosen. A portion ofthis ink film is transferred to a relief printing plate 13 having aplate impression 6, such as a Cyrel® flexographic printing plate,positioned on a drum 13′. A membrane 15, such as a perfluorinatedsulfonic acid polymer membrane in acid form which is available under thetrademark Nafion® from E. I. DuPont de Nemours and Company, positionedon a rotating drum 14 picks up the electrocatalyst coating composition11 from the relief printing plate 13, to form a relief image on themembrane. The dried relief image serves as an electrode on the membrane.This can be repeated the desired number of passes to produce the desiredthickness of the electrocatalyst coating composition 11. After drying,the membrane is then turned over for application to the opposite sidesof a catalyst coating composition 11, which may be different from thefirst applied catalyst coating composition, to form an second electrode.For example, an anode may be formed on one side of the membrane and acathode on the opposite side of the membrane.

[0046]FIG. 2 shows a continuous process employing rolls stock utilizingthree discrete printing stations to form multiple electrode layers in acontinuous fashion. As shown in FIG. 2, the membrane to be coated isunwound from roll 17, past the coating station 10 shown in FIG. 1 and adrying station 16. Additional coatings and drying can be accomplished asshown in coating stations 10 a to 10 n and drying stations 16 a and 16b, on to the coated and dried membrane from coating station 10. Anynumber of coating stations may be present between 10 a and 10 ndepending of the desired thickness of the electrode to be formed ordifferent coating compositions may be applied at each coating station toform different electrodes on the surfaces of the membrane. In coatingstations 10 a and 10 n respectively, electrocatalyst coatingcompositions 11 a and 11 n are picked up by the anilox rolls 12 a and 12n and transferred to relief printing plates 13 a and 13 n, positioned ona drum 13 a′ and 13 n′. The coated and dried membrane from coatingstation 10 n is then wound onto roll 18 past idler roll 19 as shown. Themembrane may then be turned over and run though the process again toproduce electrodes on the opposite sides. The electrocatalyst coatingcompositions at the three stations may be the same or different.Additional stations can be employed on line to print on the oppositeside of the membrane so that the catalyst coated membrane may becompleted in one pass.

[0047] The direct product of the process is a length of membrane withmultiple electrodes formed on it. Preferably, the product has the ionexchange polymer in the electrodes and in the membrane in acid form,which upon cutting, is suitable for end use without necessary processingsteps. The product can be stored in roll form which facilitate handlingand/or subsequent processing operation. For some applications,calendering can be employed to consolidate the electrode structure thatis useful for improving performance and this can easily be preformed onthe product stored in roll form. Other treatments to improve performanceare easily performed on the product stored in rolls form and can includeacid washing, e.g., nitric acid washing, heat treatments, etc.

[0048] For use in making membrane electrode structures, the direct CCMproduct of the process, after post treating if performed, is cut intothe desire size pieces and laminated to appropriate gas diffusion mediaby known techniques. The cutting operation is preferably supplied withCCM in roll form that is fed to appropriate slitting and cuttingequipment to achieve high volume manufacture.

EXAMPLES Example 1 Preparation of Alcohol Dispersions of Ion ExchangePolymer (Perfluorinated Sulfonic Acid Polymer—Acid Form)

[0049] A 3 liter rotary evaporator flask is charged with 1000 g of aperfluorinated sulfonic acid polymer dispersion (Nafion®—obtained fromDuPont), comprising 5 wt % 1100 EW perfluorinated sulfonic acid polymer(PDMOF), in 50% water-50% mixed alcohol (methanol, ethanol, 2-propanolmedia)). Rotary evaporation is commenced at 60 rpm, 15 mm Hg pressure,with the evaporation flask immersed in a 25° C. H₂O bath. A dryice/acetone bath (−80° C.) is used as the overheads condenser. Afterseveral hours of slow, steady operation, 520 gms of H₂O/mixed alcoholsis removed. As the solids level increased to a nominal 10% level, anoticeable increase in viscosity. (3→20 cps) is observed. A slowapproach to this point is necessary to avoid irreversible gelation.

[0050] After a 50 gm sample of the evaporation flask residue is removed,450 g of n-butanol is added to the evaporation flask. The clear liquidturns an opaque, milky white. The roto-vap operation is continued underthe same conditions for several more hours until a clear liquid product(436 gms) is obtained. The final measured solids content is 9.51%. Thecondensed solvent weighes 344 g, indicative of some vapor bypassing thedry ice condenser. A thin butanol layer is observed on the bulk H₂Orecovered indicating some butanol vaporization at the conditions chosen.

[0051] Repetitions of this basic procedure yield perfluorinated sulfonicacid polymer dispersions in n-butanol with solids contents of up toapproximately 13.5% by weight without gelation. Viscosities of thedispersions obtained are typically in the range of 500 to 2000 cps.(Brookfield/20 to 24° C.). Karl Fisher determinations indicate totalresidual H₂O content ranging up to 3% in the various dispersions.

[0052] In addition to the indicated perfluorinated sulfonic acid polymerdispersion (5% solids, 1100 EW), alternate starting perfluorinated ionexchange polymer suspensions can be utilized. For example, 990 EWperfluorinated sulfonic acid polymer (PDMOF) at 18% solids in 80% mixedalcohol—20% H₂O media produces similar results. Similarily, 1100 EWperfluorinated sulfonic acid polymer (PDMOF) at 50% solids in water andnominal 800 EW perfluorinated sulfonic acid polymer (POPF) at 5% solidsin mixed water/alcohol make similar alcohol dispersions using theprocedure described above.

[0053] In place of n-butanol, other alcohols that were used successfullyin the above procedure are n- and iso-amyl alcohol (n- andiso-pentanol), cyclohexanol, n-hexanol, n-heptanol, n-octanol, glycolethers and ethylene glycol.

Example 2 Preparation of Electrocatalyst Coatings Compostions

[0054] Using the above containing perfluorinated sulfonic acidpolymer/alcohol dispersions as basic component, catalyst coatingssuitable for flexographic printing of CCMs for use in fuel cells areprepared as follows:

[0055] A 13.2 wt % solids perfluorinated sulfonic acid polymer (1100EW—PDMOF) in n-butanol dispersion, prepared as described above (28.94 g)is combined with 77.31 g n-butanol. The resulting mixture is then cooleddown to ˜10° C., well below the 35° C. n-butanol flash point, by theaddition of dry ice. This serves to both lower the temperature and todisplace the ambient O₂ with the generated CO₂ gas, thus providing anadded margin of safety for the addition of the potentially pyrophoriccatalyst powder (platinum supported on carbon). To the cooled mixture,18.75 g of 60/40 C/Pt (E-Tek Corporation) is added slowly with vigorousstirring in order to wet out the powder instantly and to rapidlydissipate the heat of adsorption. (˜5 minutes total). Component amountsare calculated to yield a final solids content of 18.07 wt %. Thecalculated catalyst content on a dry solids basis is calculated to be83.07 wt %.

[0056] This mixture is then combined with 100 g of zirconia cylinders(0.25 inch×0.25 inch diameter) grinding media in a 250 cm³ mill jar. Thejar is sealed and placed on a roll mill table at ˜200 rpm at roomtemperature for 3 to 5 days. After this dispersion method the coatingcomposition is ready for testing and printing operations.

[0057] The final coating composition at nominal 18% solids has a stiff“cold cream”-like consistency that measures in the 5,000 to 20,000 cpsviscosity range by simple Brookfield methods. Simple gravimetric solidscheck give results in the 17.8 to 18.3% range. Knife coatings on heavygauge Mylar® polyester film are useful to further characterize thecoating before printing press application. A 5 mil draw knife produces aglossy black wet coating which dries (1 hr/22° C.) to a flat black, finevelvet texture, free from large particles, cracks, craters, repellenciesand streaks.

Example 3 Preparation of CCM's Utilizing Above Electocatalyst CoatingCompositions

[0058] Cyrel® flexographic printing technology (DuPont Company) is usedwith the above electrocatalyst coating composition to print directly ona variety of perfluorinated sulfonic acid polymer (acid form) filmsubstrates. The press used is a GMS Print Proof system as made by GMSCo. (Manchester, England).

[0059] The as received Cyrel® flexographic plate stock is photo-imagedvia strong UV exposure to a precision pattern by a photographic contactnegative “tool”. Exposed areas of thick photopolymer mixed film are UVcrosslinked. The unexposed areas are next washed away by the appropriateCyrel® developer solution. Left behind is the cross-linked, rubberyplate surface in sharp relief areas that act to transfer coatingcompositions in precise patterns and thicknesses to a moving filmsubstrate. The flexographic plate is mounted on a roll which in rotarymotion prints the composition on the moving substrate. After printingthe moving plate is re-coated by contacting a precision cellularapplicator roll. The cellular applicator in turn receives a fresh,metered coating composition supply from a stationary reservoir or“fountain”.

[0060] To utilize this GMS printing device, a cast perfluorinatedsulfonic acid polymer membrane (990 EW PDMOF), 1.5 mils thick,approximately 3″ wide and 10′ long is mounted on the print drum. TheCyrel® flexographic plate formulation PLS was imaged to produce three 50cm² (7×7 cm) squares aligned vertically, with each square separated by 4cm of non-image area. The plate and print drum geometry is such that 5separate plate impressions can be achieved per single rotation of theprint drum holding the membrane. In a single print drum rotation 15single impressions are made. The relative speed difference between plateand print drum is zero over the cycle eliminating scuffing, scratchesetc. The plate/film gap is adjusted to achieve plate/film contact withan additional 2 mils of plate/film compression during the initial presssetup. This is provided by adjusting the GMS anilox roll to mountedsubstrate gaps and alignment.

[0061] The anilox cell count selected was 300 lines/inch which inprinters terminology gives ˜5 billion cubic microns per sq inch. This inturn translates to a nominal 8 to 9μ wet thickness on the anilox roll.This wet film layer in part transfers to the plate. The plate in turntransfers part of this wet film layer thickness from the plate to themembrane substrate.

[0062] After printing, the plate surface is immediately re-coated byimmediate rotational contact with the anilox roll specifically chosenfor exact coating metering to Cyrel® flexographic plate surfaces. Thetypical deposition thickness of dried coating composition to themembrane substrate is about 0.7 to 0.9 microns with the 18% solidsformulation coating described above. To build increased catalyst layerthickness, with typically 0.7 to 0.9 micron dried increments, with fixedcoating/plate conditions, printing is repeated one or more times withapproximately ±0.2 mm registration on the first dried layer to yieldadditional layer(s). Additional layers can be added in successiveprint/dry applications to balance potential performance versusincremental catalyst cost. Multiple prints also tend to smooth out anydeposition non-uniformity associated with the printing process. Afterthe perfluorinated sulfonic acid polymer film is printed with one ormore layers to the desired thickness/density, it may be turned over,remounted on the drum in very precise registration with the first sideprinting, and the print process repeated to form the second side of theCCM. The mis-registration observed for as many as 12 prints/side (or 24on both sides combined) is on the order of 0.2 mm.

[0063] In this way catalyst coated membranes (CCM) have beenreproducibly machine manufactured at high speed with little or no waste.All perfluorinated sulfonic acid polymer components were used in theacid form so that there is no need for subsequent hydrolysis steps.

[0064] In addition to using cast perfluorinated sulfonic acid polymermembrane, the same catalyst printing technique can be performed on 1 and2 mil melt extruded membrane in the acid form and on 1 and 2 milpolytetrafluorethylene (PTFE)/Nafion® composite film substrates.

Example 4 Preparation of n-hexanol Based Ion Exchange Polymer DispersionElectrocatalyst Coating Composition and CCMs

[0065] Modifications of the procedures as described in Examples 1 and 2are used to prepare an 18% solids electrocatalyst coating composition.The modifications consist of replacing n-butanol with n-hexanol andreplacing the 1100 equivalent weight perfluorinated sulfonic acidpolymer starting solution with 990 equivalent weight perfluorinatedsulfonic acid polymer solution.

[0066] The resulting electrocatalyst compositions are printed on a 3inch by 10 foot strip of 2 mil thick a cast perfluorinated sulfonic acidpolymer membrane (1100 EW PDMOF) (acid form) by the method described inExample 3 except that 5 cm by 5 cm electrocatalyst impressions are made.The flexographic plate contains 6 of the 5×5 cm squares; 5 contacts ofthe plate to the above membrane film gives 30 squares. To build catalystthickness, printing is done 4 times in precise register per film side ina print/dry, print/dry series of steps. The amount of distortion causedby alcohol swelling of the film is negligible. The thickness of thefinal product was measured around the outer border of theelectrocatalyst composition squares and also at the center of thesquares (Ono-Soki Gauging system EG 225 with 1.0 micrometeraccuracy/resolution). The average substrate thickness measures 49.3micrometers. The average “substrate plus two-sided catalyst” thicknessmeasures 56.7 micrometer. By difference, the total catalyst thickness is7.4 micrometer, which is 3.7 micrometer per side. By calculation, thedried catalyst thickness per print impression is 0.93 micrometer.

Example 5 Fuel Cell Testing of CCM's

[0067] A 5×5 cm CCM prepared as in Example 4 is tested in a single cellhydrogen-air fuel cell using carbon cloth gas diffusion media sold underthe trademark ELAT® by E-Tek Corporation under the following conditions.

Experimental Conditions

[0068] Fuel Cell Clamping Force=4.0 ft-lb

[0069] Fuel Cell Temperature=80° C.

[0070] Anode Gas=Hydrogen

[0071] Anode Gas Stoichiometry=1.5 at 2 A/cm²

[0072] Anode Pressure=15 PSI

[0073] Cathode Gas=Air

[0074] Cathode Stoichiometry=2.0 at 2 A/cm²

[0075] Cathode Pressure=15 PSI

[0076] Anode and cathode gases were humidified.

Fuel Cell Performance Data

[0077] Cell Current Power Voltage Current Density Density (volts) (amps)amps/cm² watts/cm² 0.306 40.380 1.615 0.494 0.402 38.330 1.533 0.6160.499 34.580 1.383 0.690 0.595 28.380 1.135 0.675 0.708 15.280 0.6110.433 0.805 3.640 0.146 0.117 0.896 00.000 0.000 0.000

Example 6

[0078] The procedures described in Examples 1, 2 and 4 were used toprepare an 18% solids electrocatalyst coating composition based onn-hexanol and 990 equivalent weight perfluorinated sulfonic acid polymersolution with the catalyst being 60 wt % Pt on carbon as supplied byJohnson Mathey under the designation FC-60. The dry catalyst to polymerweight ratio was held at 5:1. This was printed on the same filmsubstrate as in Example 4 with the same flexographic plate on the sameequipment with the following exception: a anilox cell count used was 140lines/inch with nominal 10.5 billion cubic microns/square inch insteadof a 300 lines/inch. This provides an approximate 17 μm ink wetthickness on the anilox surface.

[0079] Film samples were taken from this process after 2, 4, 6, 8 printimpressions. The dried printed catalyst areas were analyzed underInductively Coupled Plasma (ICP) to determine platinum content/area as afunction of the number of print impressions. The results were: PrintImpressions Pt Loading (mg/cm2) 2 0.09 4 0.16 6 0.29 8 0.41

[0080] This set of data allows the following linear relationship(Y=mX+b) to be formed for this particular ink/anilox combination with98.56% R² Correlation:

Pt Loading (mg/cm2)=0.0545×(#Print Impressions)−0.035

[0081] wherein Y=Pt Loading (mg/cm2); X=#Print Impressions m(slope)=0.0545; and b (intercept)=−0.035

[0082] In this fashion, by adjusting the number of impressions, usingdifferent anilox roll sizes, altering the platinum content of thecatalyst particles, the catalyst/polymer ratio, the % solids of thecomposition etc. the catalyst loading can be adjusted and controlled ina number of different ways. The catalyst can be chosen to provide therequired quality, uniformity & productivity at the lowest overall cost.

What is claimed is:
 1. A process for manufacturing a catalyst coatedmembrane comprising: preparing an electrocatalyst coating compositioncomprising an electrocatalyst and an ion exchange polymer in a liquidmedium; and raised relief printing said electrocatalyst coatingcomposition onto a first surface of an ion exchange membrane, saidrelief-printing forming at least one electrode layer covering at least apart of said surface of said membrane.
 2. The process of claim 1 whereinsaid raised relief printing is flexographic printing.
 3. The process ofclaim 1 wherein said raised relief printing is repeated to form multipleelectrode layers covering the same part of the surface of said membrane.4. The process of claim 3 wherein said raised relief printing providesmultiple electrode layers which vary in composition among said multiplelayers.
 5. The process of claim 1 wherein said raised relief printingprovides an electrode layer with a predetermined nonuniform distributionof electrocatalyst across the electrode layer.
 6. The process of claim 1further comprising raised relief printing at least onenonelectrocatalytic coating composition to form a nonelectrocatalyticlayer over at least part of the same area of the membrane which iscovered by an electrode layer.
 7. The process of claim 6 wherein saidnonelectrocatalytic layer is an abrasion-resistant coating covering saidelectrode layer.
 8. The process of claim 1 further comprising raisedrelief printing said catalyst coating composition onto the oppositesurface of an ion exchange membrane, said relief printing forming atleast one electrode layer covering at least a part of said oppositesurface of said membrane in registration with the electrode layer onsaid first surface.
 9. The process of claim 1 wherein said ion exchangepolymer in said electrocatalyst coating composition and in said membranecomprise highly fluorinated ion exchange polymer.
 10. The process ofclaim 1 wherein said ion exchange polymer in said electrocatalystcoating composition and in said membrane comprise perfluorinated ionexchange polymer.